Computer disk drives store information on disks or platters. Typically, the information is stored on each disk in concentric tracks. The data tracks are usually divided into sectors. Information is written to and read from a storage surface(s) of a disk by a transducer. The transducer may include a read element separate from a write element, or the read and write elements may be integrated into a single read/write element. The transducer is mounted on an actuator arm capable of moving the transducer radially over the disk. Accordingly, the movement of the actuator arm allows the transducer to access different data tracks. The disk is rotated by a spindle motor at a high speed, allowing the transducer to access different sectors within each track on the disk.
The actuator arm is coupled to a motor or coarse actuator, such as a voice coil motor (VCM), to move the actuator arm such that the transducer moves radially over the disk. Operation of the coarse actuator is controlled by a servo control system. The servo control system generally performs two distinct functions: seek control and track following. The seek control function includes controllably moving the actuator arm such that the transducer is moved from an initial position to a target track position. In general, the seek function is initiated when a host computer associated with the computer disk drive issues a seek command to read data from or write data to a target track on the disk.
As the transducer approaches the target track, the servo control system initiates a settle mode to bring the transducer to rest over the target track within a selected settle threshold, such as a percentage of the track width from track center. Thereafter, the servo control system enters the track following mode wherein the transducer is maintained at a desired position with respect to the target track (e.g., over a centerline of the track) until desired data transfers are complete and another seek is performed.
The ability to precisely position a transducer with respect to a track being followed has become increasingly important, as data and track densities in disk drives have increased. In particular, the space between adjacent tracks has become increasingly small, and the tracks themselves have become increasingly narrow. In order to increase the precision with which a transducer may be positioned with respect to a track during track following, an articulated actuator arm may be used. In general, the angle of the distal portion, or second stage, of the actuator arm with respect to the main portion, or first stage, of the actuator arm is controlled by a micro actuator. By operating the micro actuator to introduce small changes in the position of the transducer with respect to a track being followed, the accuracy of track following operations may be increased.
Because the location of the transducer is a combination of the contributions of the coarse actuator and the micro actuator, the position of the micro actuator within its relatively small range of motion typically isn't directly observable. Instead, the current position and response of the micro actuator to movement commands is usually estimated through a model of the micro actuator. Accordingly, the accuracy of the estimated response of the micro actuator to movement commands can substantially affect the precision with which the transducer can be positioned relative to a track.